1. Field of the Invention
The present invention relates to a rotational angle sensor for determining the absolute angular position of a body such as a vehicle steering shaft that can rotate through several rotations (i.e., rotatable by more than 360°) in which the sensor includes first and second measuring elements each having a magnet which rotates relative to a respective magnetic sensor element as the body rotates with the magnets being driven at different gear ratios such that the difference between the rotational angle positions of the magnets results in a “beat angle” in which more than one beat angle period is contained within the angular measuring range of the rotational angle sensor and the rotational angle position of the magnets in the beat angle periods is different.
2. Background Art
Rotational angle sensors determine the instantaneous rotational angle position of a rotatably supported body. For example, a steering angle sensor determines the absolute angular position of the steering shaft of a vehicle. Steering angle sensors may operate optoelectronically or magnetically.
An optoelectronic steering angle sensor includes a code disk in mechanical linkage with the shaft to rotate therewith. The code disk circumferentially surrounds and radially extends from the shaft. The code disk has a certain radial extension so that necessary angle coding information for enabling the sensor to determine the absolute angular position of the shaft within an angular measuring range extending over several revolutions can be accommodated on the code disk.
U.S. Pat. No. 5,930,905 (corresponding to EP 0 877 916 A1), which is hereby incorporated by reference, describes a magnetic steering angle sensor having a drive wheel and first and second code wheels. The drive wheel is connected to the shaft to rotate therewith and includes a gear ring. Each code wheel includes a gear ring and a magnet whose magnetic field orientation is a measure of the rotational angle position of the code wheel. First and second fixedly placed magneto-sensitive elements, such as GMR sensors, AMR sensors, or Hall effect sensors, respectively monitor the magnetization of the magnets to determine the rotational angle position of the code wheels. The gear rings of the code wheels engage with the gear ring of the drive wheel such that the code wheels rotate as the shaft rotates. The absolute angular position of the shaft within the angular measuring range of the steering angle sensor extending over several revolutions of the shaft is determined by application of the Nonius principle. In particular, the code wheels are driven at different gear ratios than the drive wheel and have a smaller number of teeth than the drive wheel. The gear reduction is such that one code wheel rotates 360° n times and the other code wheel rotates 360° n+1 times over the angular measuring range, where n is greater than the number of revolutions of the angular measuring range. For example, the first code wheel rotates nine times (i.e., 3240°) and the second code wheel rotates ten times (i.e., 3600°) over the angular measuring range which extends over four revolutions (i.e., 1440°) of the shaft. The difference between the rotational angle positions of the code wheels results in a “beat angle”. The beat angle has a constant slope over the angular measuring range with the value of the beat angle at one end of the angular measuring range being 0 and the value of the beat angle at the other end of the angular measuring range being 360°. In order to determine the particular revolution of a code wheel, the beat angle is scaled and the rotational angle position associated with the code wheel is subtracted from the scaled beat angle. The scaling of the beat angle is such that subtracting the rotational angle position from the scaled beat angle results in individual horizontal plot segments, each of which defines an independent level. Each level defines one revolution of this code wheel so that the revolution information for determining the absolute angular position of the shaft may be obtained. A problem with the magnetic steering angle sensor is that magnetic interference results in a reduction of the effectively usable measuring range.
A plausibility check may be performed to verify the determined absolute angular position of the shaft. For example, the plausibility check involves the angle position of the vehicle wheels. The measurement accuracy of the wheel angle determination may be within the range of the angular error that occurs when an adjacent revolution is selected in error. This results in error in the angle determinations within the scope of the plausibility check of the determined absolute angular position.
U.S. Pat. No. 6,745,116 (corresponding to DE 103 43 543 B3), which is hereby incorporated by reference, describes a similar magnetic steering angle sensor. The angular measuring range of the sensor contains multiple beat angle periods with the cyclic periods of the two magnets being selected such that the smallest common multiple of the cyclic periods is equal to a predetermined steering angle of a vehicle. This results in very different drive ratios for the magnets and thus results in very different rotational speeds.